Novel Na-silicates CO2 Sorbents from Fly Ash

Potassium Silicate as Low-Temperature Binder in 3D-Printed Porous Structures for CO2 Separation

Activated carbon sorbents were directly 3D-printed into highly adaptable monolithic/multi-channel systems by using potassium silicate as a low-temperature binder. By employing emerging 3D-printing technologies, monolithic structured sorbents were printed and fully characterized using N₂, Ar, and CO₂-sorption and Hg-intrusion porosimetry. The CO₂-capture performance and the required temperature for active-site regeneration were evaluated by thermogravimetric analysis-looping experiments. A mechanically stable activated carbon sorbent was developed with an increased carbon capture performance, even when a room-temperature regeneration by N₂ purging was applied. Although the CO₂ uptake slightly dropped after several cycles due to incomplete recovery at room temperature, a capacity increase of 25% was observed in comparison with the original activated carbon powder. To improve the recovery of the active sorbent, an optimization of the desorption step was performed by increasing the regeneration temperature up to 150 °C. This resulted in a CO₂ uptake of the composite material of 0.76 mmol/g, almost tripling the working capacity of the original activated carbon powder (0.28 mmol/g). An in situ X-ray diffraction study was carried out to confirm the proposed sorption mechanism, indicating the presence of potassium bicarbonates and confirming the combination of physisorption and chemisorption in our composites. Finally, the structured adsorbent was heated homogeneously by applying a current through the monolith. These results describe the development of a new type of 3D-printed regenerable CO₂ sorbents by using potassium silicate as a low-temperature binder, providing high mechanical strength, good chemical and thermal stability, and improving the total CO₂ capacity. Moreover, the developed monolith is showing a homogeneous resistivity, leading to uniform Joule heating of the CO₂ adsorbent.

A theoretical study on CO2 at Li4SiO4 and Li3NaSiO4 surfaces

Lithium silicates have attracted great attention in recent years due to their potential use as high-temperature (450-700 °C) sorbents for CO₂ capture. Lithium orthosilicate (Li₄SiO₄) can theoretically adsorb CO₂ in amounts up to 0.36 g CO₂ per g Li₄SiO₄. The development of new Li₄SiO₄-based sorbents is hindered by a lack of knowledge of the mechanisms ruling CO₂ adsorption on Li₄SiO₄, especially for eutectic mixtures. In this work, the structural, electronic, thermodynamic and CO₂ capture properties of monoclinic phases of Li₄SiO₄ and a binary (Li₃NaSiO₄) eutectic mixture are investigated using density functional theory. The properties of the bulk crystal phases as well as of the relevant surfaces are analysed. Likewise, the results for CO₂-lithium silicates indicate that CO₂ is strongly adsorbed on the oxygen sites of both sorbents through chemisorption, causing an alteration not only in the chemical structure and atomic charges of the gas, as reflected by both the angles and bond distances as well as atomic charges, but also in the cell parameters of the Li₄SiO₄ and Li₃NaSiO₄ systems, especially in Li₄SiO₄(001) and Li₃NaSiO₄(010) surfaces. The results confirm strong adsorption of CO₂ molecules on all the considered surfaces and materials followed by CO₂ activation as inferred from CO₂ bending, bond elongation and surface to CO₂ charge transfer, indicating CO₂ chemisorption for all cases. The Li₄SiO₄ and Li₃NaSiO₄ surfaces may be proposed as suitable sorbents for CO₂ capture in wide temperature ranges.

Alkali carbonates promote CO2 capture by sodium orthosilicate

The C₂O₅²⁻ combined with alkali cations are the intermediates during the CO₂ uptake with Na₄SiO₄ and alkali carbonates.

Potassium-based sorbents from fly ash for high-temperature CO2 capture

Potassium-fly ash (K-FA) sorbents were investigated for high-temperature CO₂ sorption. K-FAs were synthesised using coal fly ash as source of silica and aluminium. The synthesised materials were also mixed with Li₂CO₃ and Ca(OH)₂ to evaluate their effect on CO₂ capture. Temperature strongly affected the performance of the K-FA sorbents, resulting in a CO₂ uptake of 1.45 mmol CO₂/g sorbent for K-FA 1:1 at 700 °C. The CO₂ sorption was enhanced by the presence of Li₂CO₃ (10 wt%), with the K-FA 1:1 capturing 2.38 mmol CO₂/g sorbent at 700 °C in 5 min. This sorption was found to be similar to previously developed Li-Na-FA (2.54 mmol/g) and Li-FA (2.4 mmol/g) sorbents. The presence of 10 % Li₂CO₃ also accelerated sorption and desorption. The results suggest that the increased uptake of CO₂ and faster reaction rates in presence of K-FA can be ascribed to the formation of K-Li eutectic phase, which favours the diffusion of potassium and CO₂ in the material matrix. The cyclic experiments showed that the K-FA materials maintained stable CO₂ uptake and reaction rates over 10 cycles.

Bifunctional application of sodium cobaltate as a catalyst and captor through CO oxidation and subsequent CO2 chemisorption processes

Sodium cobaltate works as a bifunctional material, in the catalysis of CO oxidation and subsequent CO₂ chemisorption.